Under-frequency Load Shedding Scheme Research Articles

Impact of Synchrophasor Estimation Algorithms in ROCOF-Based Under-Frequency Load-Shedding

This paper investigates the impact of synchrophasor estimation algorithms in Under-Frequency Load-Shedding (UFLS) and Load-Restoration (LR) schemes, relying on frequency and Rate-of-Change-of-Frequency (ROCOF) measurements produced by Phasor Measurement Units (PMUs). We compare two consolidated window-based synchrophasor estimation algorithms, as representative approaches based on static and dynamic signal models, with a focus on the appropriateness of utilizing PMU-based ROCOF measurements. The performance of the proposed relaying scheme is assessed by means of a Real-Time Simulator implementing the time-domain full-replica dynamic model of the IEEE 39-Bus power system.

An improved adaptive wide-area load shedding scheme for voltage and frequency stability of power systems

Conventional under-frequency and under-voltage load shedding schemes are known as effective approaches for load shedding purposes. Generally, these two schemes are deployed independently within which the combinatorial disturbances are overlooked. Contributing to this context, the ongoing study raises a high concern on developing a generalized wide-area adaptive load shedding scheme. In order to preserve the voltage and frequency stabilities, the established approach deploys the variations of voltage, frequency, active, and reactive power in all buses, simultaneously. More specifically, the reactive power variation on voltage stability is incorporated in the proposed load shedding index. This idea improves the performance of the proposed scheme, significantly. Infield phasor measurement units are establishing a wide-area basis, enabling the proposed approach in modern control centers. Extensive numerical studies are devised to assess the performance of the proposed approach encountering various disturbances. The obtained results are discussed in depth.

Improved UFLS with consideration of power deficit during shedding process and flexible load selection

This study presents an improved under-frequency load shedding (UFLS) scheme that can detect power deficit during the shedding process and accordingly adjust the amount of load shedding. This is achieved by continuous monitoring of the overshooting signal of the second frequency derivative of the centre of inertia. Once detected, an equivalent system inertia constant is estimated in order to quantify the new power deficit. The scheme is also equipped with an optimisation algorithm to determine the best combination of loads that is close to the amount of power deficit, which minimises frequency overshoot/undershoot. The optimisation technique selected for this work is based on particle swarm optimisation. The performance of the proposed UFLS scheme was validated using a modified IEEE 33 bus with two mini-hydro generators and one full converter wind turbine. The system and the proposed UFLS was modelled and simulated in PSCAD/EMTDC software. The results confirmed that the proposed scheme is capable of shedding loads with minimum undershoot/overshoot, and detect and estimate a new power deficit during load shedding. The results reported by the proposed scheme proved to be significantly better than those reported by conventional and adaptive load shedding schemes.

A Smart Under-Frequency Load Shedding Scheme based on Takagi-Sugeno Fuzzy Inference System and Flexible Load Priority

This paper proposes a new smart under frequency load shedding (UFLS) scheme, based on Takagi-Sugeno (TS) fuzzy inference system and flexible load priority. The proposed scheme consists of two parts. First part consists of fuzzy load shed amount estimation module (FLSAEM) which uses TS-fuzzy to estimate the amount of load shed and sends its value to accurate load shedding module (ALSM) to perform accurate load shedding using flexible load priority. The performance of the proposed scheme is tested for intentional islanding case and increment of sudden load in the system. Moreover, the response of the proposed scheme is compared with adaptive UFLS scheme to highlight its advantages. The simulation results show that the proposed UFLS scheme provides the accurate load shedding due to advantage of flexible priority whereas adaptive UFLS scheme due to fixed load priority does not succeed to achieve accurate load shedding.

A Centralized UFLS Scheme Using Decision Trees For Small Isolated Power Systems

This paper presents a centralized underfrequency load-shedding (UFLS) scheme using a decision tree to decide whether and how much load needs to be shed. UFLS schemes play an important role in protecting the integrity of small isolated power systems. Centralized UFLS schemes continuously measure and process critical system variables and act on the feeder breakers according to the system state. The decision determines the optimal amount of load to be shed by means of dichotomous tests applied to measured input variables related to the pre- and post-contingency state of the system. The applicability of the centralized UFLS scheme is shown for a Spanish small isolated power system.

Optimal System Frequency Response Model and UFLS Schemes for a Small Receiving-End Power System after Islanding

Large frequency deviations after islanding are exceedingly critical in small receiving-end power systems. The under-frequency load shedding (UFLS) scheme is an efficient protection step for preventing system black outs. It is very important to get an exact model to design the UFLS schemes. In this paper, an optimization model to achieve the system frequency response (SFR) model either from the full-scale power system or from test records was proposed. The optimized SFR model took into account the response of governors-prime movers and the dynamic characteristics of loads developed in the modern power system. Then the UFLS schemes were designed via the optimized SFR model and particle swarm optimization (PSO) method. The time-domain simulation with the actual small receiving-end power system was presented to investigate the validity of the presented model and the developed technique.

An Under-frequency Load Shedding Scheme with Continuous Load Control Proportional to Frequency Deviation

Under frequency load shedding (UFLS) is an important measure to tackle with frequency drop caused by load-generation imbalance. In existing schemes, loads are shed by relays in a discontinuous way, which is the major reason leading to under-shedding and over-shedding problems. With the application of power electronics technology, some loads can be controlled continuously, and it is possible to improve the UFSL with continuous loads. This paper proposes an UFLS scheme by shedding loads continuously. The load shedding amount is proportional to frequency deviation before frequency reaches its minimum during transient process. The feasibility of the proposed scheme is analysed with analytical system frequency response model. The impacts of governor droop, system inertia, and frequency threshold on the performance of the proposed UFLS scheme are discussed. Cases are demonstrated to validate the proposed scheme by comparing it with conventional UFLS schemes.

Automatic under-frequency load shedding mal-operation in power systems with high wind power penetration

Countries with a limited interconnection capacity suffer substantial frequency variations after large incidents so they use automatic under-frequency load shedding schemes to arrest the frequency decay. Some of these countries such as Portugal, Spain and Ireland also have very high wind penetrations. This can cause additional frequency excursions due to generation time variability but also to the fact that variable speed wind turbines do not add directly their inertia to the power system. Thus several transmission system operators have announced new grid codes requiring wind turbines to provide frequency response.In some scenarios, however, wind energy support may be detrimental to frequency control because it generates an extra energy that reduces decay and derivative but that cannot be maintained over time. These lower values of frequency decay and derivative are currently expected after a reduced incident or when conventional generation, which can maintain the extra generation, provides frequency support, so lead to low or no load shedding. This paper has studied, in particular, the effect of wind generation emulating inertia. A reduction of frequency derivative is achieved, which looks positive at first, but in some cases leads to initial smaller load shedding than the incident requires. A reduced frequency derivative triggers less under-frequency relays as if there were a significant amount of conventional generation that is online. However, this generation has been substituted by wind generation emulating inertia, and as it can maintain extra generation over time, the frequency continues to decay until the shedding of the next load step. As a result there is an excessive frequency deviation and an incorrect load shedding for the magnitude of the initial disturbance. In order to prevent this problem, automatic under frequency load shedding settings may need readjustment when a large amount of wind generation provides frequency support.

Coordinated Scheme of Under-Frequency Load Shedding with Intelligent Appliances in a Cyber Physical Power System

The construction of a cyber physical system in a power grid provides more potential control strategies for the power grid. With the rapid employment of intelligent terminal equipment (e.g., smart meters and intelligent appliances) in the environment of a smart grid, abundant dynamic response information could be introduced to support a secure and stable power system. Combining demand response technology with the traditional under-frequency load shedding (UFLS) scheme, a new UFLS strategy-determining method involving intelligent appliances is put forward to achieve the coordinated control of quick response resources and the traditional control resources. Based on this method, intelligent appliances can be used to meet the regulatory requirements of system operation in advance and prevent significant frequency drop, thereby improving the flexibility and stability of the system. Time-domain simulation verifies the effectiveness of the scheme, which is able to mitigate frequency drop and reduce the amount of load shedding.

Improved Load-Shedding Scheme Considering Distributed Generation

With high penetration of distributed generation (DG), the conventional underfrequency load shedding (UFLS) faces many challenges and may not perform as expected. This paper proposes new UFLS schemes, which are designed to overcome the shortcomings of the traditional load-shedding scheme. These schemes utilize directional relays, power flow through feeders, wind, and photovoltaic (PV) measurements to optimally select the feeders to be disconnected during load shedding such that DG disconnection is minimized while disconnecting the required amount of consumption. These different UFLS schemes are compared in terms of frequency response, amount of consumption, and DG disconnected during load shedding.

Determinación de la Respuesta Dinámica y Estacionaria de la Frecuencia frente a Desbalances de Potencia Producidos por Perturbaciones en la Red

En este trabajo se propone una nueva metodología para determinar la respuesta dinámica y estacionaria de la frecuencia, ante desbalances de generación–carga producidos por perturbaciones en el Sistema Eléctrico de Potencia (SEP). Partiendo del modelo equivalente del sistema de control potencia frecuencia, se derivan expresiones analíticas para calcular la máxima desviación transitoria de la frecuencia, el instante en que se produce, y la cantidad de carga a desconectar por actuación del Esquema de Alivio de Carga (EAC). Con este fi n, se representa al conjunto regulador de velocidad–turbina de cada generador mediante un modelo reducido de 1er orden. En el presente trabajo, el modelo ha sido extendido para representar reguladores de unidades hidráulicas de diferente tecnología. Adicionalmente, la metodología permite determinar el valor estacionario de la frecuencia, luego de la actuación de la Regulación Primaria de Frecuencia (RPF), considerando la carga desconectada por actuación del EAC y/o el déficit de reserva rotante en unidades en RPF. Finalmente, la metodología propuesta es aplicada en dos sistemas de prueba, mostrando una excelente capacidad para estimar, con alta precisión, la respuesta dinámica y estacionaria de la frecuencia ante desbalances de potencia activa.

Reducing Under-Frequency Load Shedding in Isolated Power Systems Using Neural Networks. Gran Canaria: A Case Study

Small isolated power systems often experience generator outages, which are responsible for the activation of the under-frequency load shedding scheme with the corresponding negative impact on electricity consumers and, hence, market loss. There are three main causes of this problem: the power system's low inertia, the speed governors' low capacity, and a poor size-ratio between generator and system. The most extensive research line in this area is focused on the optimization of the load shedding scheme, which is a partial solution. Another research line is presented to solve the problem from the point of view of the system operator. This paper proposes an online method to predict and correct possible load shedding by redistributing load dispatching. This proposal uses artificial intelligence techniques, in particular neural networks, and a special-purpose power system simulator. In order to evaluate the proposal, the achieved solution is applied to a real case study: the island of Gran Canaria. This application shows the improvement that might be achieved by implementing this simple method. The method proposed in this paper is strongly recommended for regions that have suitable geographical sites as well as energy problems similar to those of the Canary Islands (see tech. rep. “Map of the Canary Islands Power Systems” by Red Electrica de Espana).

Sizing and Controller Setting of Ultracapacitors for Frequency Stability Enhancement of Small Isolated Power Systems

This paper studies the impact of the size and controller settings of an ultracapacitor (UC) on frequency stability. Underfrequency load-shedding (UFLS) schemes play an important role in protecting the system against frequency instability and their performance is used as a measure for the enhancement of frequency stability thanks to the UC. To this end, a simplified but still accurate model of the UC is developed. In addition, a meaningful procedure is proposed to size the UC's power and energy storage capacities in function of the system parameters and operation constraints in order to improve frequency stability. Finally, the impact of the UC on the UFLS scheme is studied for many generation dispatch scenarios and for all possible single generating unit outages of a Spanish isolated power system. An analysis with respect to the UC's control parameters and its size in terms of power and energy storage capacities is carried out, too. Appropriate control parameter settings improve the UC's impact on frequency stability.

A UFLS Scheme for Small Isolated Power Systems Using Rate-of-Change of Frequency

This letter presents a centralized underfrequency load-shedding (UFLS) scheme using the initial rate-of-change of frequency (ROCOF) to decide whether and how much load needs to be shed. The applicability of the centralized UFLS scheme is shown for a Spanish small isolated power system.

Predictive underfrequency load shedding scheme for islanded power systems with renewable generation

Underfrequency load shedding is one of the most important protection mechanisms in a power system. However, in the majority of power systems it has remained unchanged for decades, despite the advances in computer and communication technologies. Wide Area Monitoring System proved to be very useful and reliable in the last few years and this is why much effort is given into development of Wide Area Monitoring, Protection and Control concepts. Adaptive underfrequency load shedding is certainly a very suitable candidate for the task. In order to accelerate changes in this area, a proposal for an adaptive scheme has been developed, which is a good option from both the technical and economic points of view. In contrast to majority of adaptive schemes, it handles problems of active-power deficit estimation and its variations during the transient due to voltage dependent loads differently, i.e., by on-line forecasting the operating point trajectory in a phase space. The scheme has been tested on previously proven (by comparison to WAMS measurements) dynamic model of a part of a Slovenian power system and the results indicate a large improvement compared to the traditional scheme.

An Underfrequency Load Shedding Scheme for Hybrid and Multiarea Power Systems

The purpose of underfrequency load shedding (UFLS) is to maintain the frequency of an interconnected or island power system within permissible limits in case of severe power deficits. Thus, the proper performance of UFLS scheme is of great importance. In hybrid power systems, disconnecting and connecting the power generation sources and loads result in variations in the system parameters. Moreover, the output power of some distributed energy resources, such as photovoltaic (PV) or wind turbine generator (WTG), might change during the load shedding process. These factors may affect the performance of the load shedding scheme and decrease the reliability of power system. Hence, in this paper, a UFLS scheme is proposed that has two advantages: 1) it is independent of power system parameters; and 2) it considers power generation variations during the load shedding process. To verify the effectiveness of the proposed load shedding method, simulation studies are carried out on a hybrid power system including PV, WTG, fuel cell, diesel generator, and battery energy storage system. Finally, the performance of the proposed method on a multiarea power system is evaluated.

Hardware Implementation of an Automatic Adaptive Centralized Underfrequency Load Shedding Scheme

The underfrequency load shedding (UFLS) mostly used in industry is a decentralized deterministic scheme designed to shed a prespecified amount of load after a predetermined time delay. It sheds the same amount of load from the same location irrespective of how fast the frequency drops and without consideration of the disturbance location or dip in bus voltage. Recent studies focused on adaptive UFLS, but these studies are still based on software simulation. This study presents an implementation of a real-time centralized adaptive UFLS scheme using industry-grade hardware. It estimates the amount of load to be shed based on the rate of frequency decline and distributes the load to be shed among the load buses based on the voltage dip at these buses. The UFLS in this study is implemented using a real-time digital simulator, phasor measurement units embedded in the relays, a global positioning system clock, and a synchrophasor vector processor. The load is modelled as a mixture of dynamic and static load. The implemented scheme restored the system frequency and voltage. The results emphasize its adaptability and suitability for implementation in industry.

Impact of Large-scale Wind Power Integration on the UFLS Scheme

This paper analyzes some major factors for the impact of wind power integration on power system dynamic frequency characteristics in theory. Combining with an actual grid, test scenarios with various scales of doubly fed induction generator (DFIG)-based wind power integration are formulated. Through simulation in the scenarios with FASTEST software, the impacts on the action of under frequency load shedding (UFLS) scheme are displayed in two aspects, the rate of change of frequency (ROCOF) and wind turbine generator own protection. Simulation result shows that the ROCOF will not affect the lock action of UFLS devices in the test system of this paper. Wind turbine generator protection may worsen frequency responses, so it is proposed to be in harmony with the design of UFLS scheme.

Adaptive Decentralized Under-Frequency Load Shedding for Islanded Smart Distribution Networks

Frequency stability of islanded smart distribution networks with peer-to-peer (P2P) controlled distributed generators (DGs) has attracted special attention recently. Using power line communication (PLC) technology, a multi-agent system (MAS)-based, decentralized, under-frequency load shedding (UFLS) scheme is investigated in this study for smart distribution networks with the communication constraint that each agent can only communicate with its neighboring agents. The proposed scheme uses the two-layer nearest neighbor consensus algorithm (NNCA) to overcome the drawback of the leader–follower consensus algorithm and guarantee the implementation of a decentralized UFLS. Based on the global information (i.e., the magnitude of the total active power imbalance) discovered in the first layer of the NNCA, the multi-stage UFLS can be executed by the second layer of the NNCA. Simulation results demonstrate that the proposed scheme can effectively implement the decentralized UFLS while maintaining the frequency stability of an islanded smart distribution network.

An underfrequency load shedding scheme for islanded microgrids

The underfrequency load shedding scheme is an important protective schemes, responsible for maintaining frequency of power systems within permissible limits in case of severe power deficits and as such, its proper performance is of great interest. In microgirds, disconnecting/connecting power generation sources and loads result in variations in the system parameters. Moreover, the output power of some distributed energy resources such as photovoltaic/wind turbine generators, might change during load shedding process and consequently affect the performance. Hence, an underfrequency load shedding scheme is proposed here which is not only independent of the microgrid parameters, but also considers power generation variations during the process. A microgrid power system with several distributed energy resources is adopted to demonstrate the effectiveness of the proposed method.